The invention relates to an optical branch coupler for splitting a light beam into two further light beams or for combining two light beams into a single light beam. The coupler comprises a housing which accommodates an optical element. The optical element is arranged between a first and a second side wall. It conducts a part of a light beam which is radiated through a first opening in the first wall to a second opening in the second wall. The optical element also reflects a further part of the light beam through an opening in a third side wall which extends at a right angle to the substantially parallel first and second walls. The optical element is arranged to be adjustable.
An optical branch coupler of this kind is known from published United Kingdom patent application No. 2,012,984 (corresponding to German Offenlegungsschrift No. 29.00.895). The device described therein comprises an optical distribution element for distributing an incoming light beam between two exits. In order to obtain a desirable transmission of light from an entrance to an exit with a minimal loss of light, the optical element should be adjustable. However, this patent application does not disclose how the distribution element is to be adjusted.